During the formation of semiconductor devices, such as dynamic random access memories (DRAMs), static random access memories (SRAMs), microprocessors, etc., insulating layers, such as silicon dioxide, phosphorous doped silicon dioxide, or other doped oxide, are used to electrically separate conductive layers, such as doped polycrystalline silicon, doped silicon, aluminum, refractory metal silicides etc. It is often required that the conductive layers be interconnected through holes in the insulating layer. Such holes are commonly referred to as contact holes, i.e., when the hole extends through an insulating layer to an active device area, or vias, i.e., when the hole extends through an insulating layer between two conductive layers. The profile of a hole is of particular importance such that it exhibits specific characteristics when the contact hole or via is provided or filled with a conductive material. For example, many holes are high aspect ratio holes.
It is known to utilize plasmas containing fluorocarbons or hydrofluorocarbons to etch oxides relative to underlying silicon containing layers. For example, plasmas containing CF.sub.4 have been used to perform such an etch. Using fluorocarbon or hydrofluorocarbon containing plasmas provides a means of selectively etching oxide films against an underlying silicon containing layer, i.e., the etching of the oxide film down to the underlying silicon layer without significantly etching the underlying silicon containing layer. In such a case, a high oxide to silicon etch rate ratio is required.
The mechanism responsible for fluorocarbons to accomplish high silicon dioxide to silicon etch rate selectivity involves the combination of at least two phenomena. First, the deposition of nonvolatile residue, e.g., a polymeric containing residue, is formed on various surfaces during the etching process, and second, the oxygen from the etching of the oxide in the process performs a particular role. While carbon containing residues are found to deposit on all surfaces inside an etch chamber containing fluorocarbon or hydrofluorocarbon plasmas, less accumulation is observed to occur on oxide surfaces, e.g., doped silicon dioxide, than on non-oxide surfaces, e.g., silicon containing surfaces such as silicon nitride, doped silicon, or polysilicon.
Carbon containing residues or polymeric residues deposit on surfaces in a hole (e.g., walls, bottom) when fluorocarbon discharges are present in several ways. One way involves the dissociation of fluorocarbon radicals upon being absorbed on a surface. Less residue accumulates on silicon dioxide surfaces because some of the carbon combines with the oxygen of the oxide being etched to form carbon monoxide or carbon dioxide, which are volatile. This in turn allows the silicon dioxide layer to continue to be etched under certain conditions when etching of other materials has ceased because of the formed nonvolatile residue. If a nonvolatile layer (e.g., carbon residue) deposits on a surface during etching, and it is not removed, etching will cease.
Further, with the use of a fluorocarbon or hydrofluorocarbon containing plasma, if the etching mechanism proceeds strictly by chemical action (e.g., the reaction of silicon with fluorine atoms generated by the plasma to form SiF.sub.4), then only isotropic etching is accomplished which provides no advantage over wet etching of such silicon dioxide or oxides to form contact holes or vias. However, plasmas generated using fluorocarbons or hydrofluorocarbons allow for the ability to provide an anisotropic etch which is believed to depend in some way or another on the bombardment of the etched surface with energetic ions.
For example, in a typical silicon dioxide etching process, to provide a contact hole or via on a wafer, incident energetic particles generally arrive in a direction perpendicular to the wafer surface, hence they strike the bottom surfaces of the etched features. In processes, such as those using fluorocarbon or hydrofluorocarbon containing plasmas in which polymer deposition on the side wall and the bottom surface of the contact hole or via being etched occur simultaneously with the etching of the oxide, (i.e., nonvolatile polymer layers (also commonly referred to as surface inhibiting or blocking layers) that deposit on the surfaces being etched), surfaces not struck by the ions do not have the blocking layer removed and hence are protected against etching by the reactive gas. As such, etching is performed in a direction perpendicular to the wafer surface more quickly than etching of the side walls.
However, an "etch stop" phenomenon with respect to high aspect ratio features, such as contact holes and vias, is problematic. For example, as shown in FIG. 1, a substrate assembly 12 has an oxide layer 14 formed thereon. The oxide layer 14 is patterned using a mask layer 16 which defines the contact hole or via 18 through the oxide layer 14. With the mask layer 16 patterned, the contact hole or via 18 is etched using a fluorocarbon or hydrofluorocarbon plasma 22. As shown in FIG. 1, during the etching of the contact hole or via 18 with species (including ions 23) extracted from plasma 22, a nonvolatile polymeric residual layer 20 is formed on the side walls 19 and bottom surface 21 of the contact hole or via 18 due to carbon containing neutral species 24 resulting from the etch process. Such deposition of the polymeric residual layer 20 and etching of the oxide layer 14 occur simultaneously. When high aspect ratio features are etched, the etch rate and etch chemistry varies with the aspect ratio (or depth) of the feature. Often in contact hole or via etching the process starts out etching normally but at a certain aspect ratio the etching action undesirably stops, i.e., etch stop phenomena.
Therefore, a major problem in etching high aspect ratio contact holes and vias in oxides is that the etch chemistry changes with changing aspect ratio of the etched hole resulting in premature etch stop. This effect is most severe in the oxide contact hole and via etch processes because of the need to use a chemistry in which the etching of the oxide and the deposition of a polymeric residual material are taking place simultaneously. Because of the polymer deposition, the etch process may stop spontaneously well before the desired oxide is etched completely through, i.e., etch stop.
It should be recognized that rare gases are often mixed with feed gases in etch processes. Argon and helium are the most commonly used rare gases. Such use of rare gases is typically to dilute the chemical species and to stabilize the plasma being generated. In particular, various articles have discussed silicon dioxide contact etching with rare gases added to fluorocarbon feed gases. However, such rare gas additions have been limited to argon and helium in the etching of contact holes and vias. Further, the most common reasons for rare gas additions to a plasma are to improve the behavior of the plasma glow discharge. For example, electronegative gases, such as chlorine, SF.sub.6, etc., form negative ions which reduce the electron concentration and causes the discharge to be unstable for which rare gases provide benefit. Further, for example, sometimes discharge ignition can be made much easier with rare gas additions. If the reactive gas prefers to be in liquid form (e.g., Br.sub.2), rare gases are also sometimes used as carrier gases to carry the reactive gas to the plasma generation chamber. Rare gases, usually argon, have also been frequently injected at the 1-5% level as standards for an optical emission spectroscopy calibration process known as actinometry.
The present invention addresses the problems as indicated above, in particular, the etch stop phenomena in the etching of contact holes or vias in oxides. However, other advantages will become apparent to those skilled in the art from the following Detailed Description read in conjunction with the appended claims and the attached Figures.